It is known that a mechanical connection between two components made of resin material is performed by welding using laser irradiation. For instance, a resin-made throttle valve and a resin-made throttle shaft included in a throttle valve device of an internal combustion engine are connected with each other by using a laser welding device (refer to Patent Document 1). In this throttle valve device, the throttle valve being a first component as a welded object is formed of resin material that has a tendency to transmit a laser beam. By contrast, the throttle shaft being a second component is formed of resin material that has a tendency to absorb a laser beam. Thus, when the laser beam is radiated through the throttle valve to the throttle shaft, the surface of the throttle shaft contacting with the throttle valve is heated up. Their boundary areas are thereby melted, so that these components are welded to each other.
Here, the welded portion does not appear as the surface of the final product, so that, for instance, presence or absence of the welded portion cannot be visually observed from an outside of the final product. This poses inexpediency. To solve this inexpediency, a component that transmits the laser beam is provided with an opening, through which the presence and absence of the welded portion can be directly visually observed (refer to Patent Document 2).                Patent Document 1: JP-2003-13751 A (Pages 2, 3; FIGS. 1 to 9)        Patent Document 2: JP-H8-86945 A (Pages 2, 3; FIGS. 1 to 3)        
In the above method disclosed in Patent Document 2, necessity of having an opening in the welded portion within the component that is disposed at a position from which the laser beam is radiated, eventually poses a problem that decreases a mechanical strength of the final product. In particular, decreasing the mechanical strength is not accepted by a throttle valve device of an internal combustion engine that undergoes thermal stresses derived from significant mechanical vibrations or rapid thermal fluctuations.
Further, since the welded portion is observed through the opening, observing the welded portion is sometimes difficult depending on lighting conditions. This involves difficulty in confirming, thereby possibly resulting in missing confirming.